The production processes of LDPE, HDPE and LLDPE are summarised in “Handbook of Polyethylene” by Andrew Peacock (2000; Dekker; ISBN 0824795466) at pages 43-66. The catalysts can be divided in three different subclasses including Ziegler Natta catalysts, Phillips catalysts and single site catalysts. The various processes may be divided into solution polymerisation processes employing homogeneous (soluble) catalysts and processes employing supported (heterogeneous) catalysts. The latter processes include both slurry and gas phase processes.
The polymerisation of ethylene with supported chromium based catalysts is disclosed by Kevin Cann in “Comparison of silyl chromate and chromium oxide based olefin polymerisation catalysts” (Macromolecular Symp, 2004, 213, 29-36).
The chromium oxide based catalyst, which is commonly referred to in the literature as “the Phillips catalyst”, can be obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere. The chromium oxide catalysis and the ethylene polymerisation with this specific catalyst are disclosed in “Handbook of Polyethylene” by Andrew Peacock at pages 61-64.
The silyl chromate catalyst consists of silylchromate (bis-triphenylsilyl chromate) absorbed on dehydrated silica and subsequently reduced with for example diethylaluminum ethoxide. The silylchromate catalysts are more expensive than chromium oxide catalysts.
Although similar in structure the oxo and triphenylsilyl chromate catalyst perform very differently in polymerisation reactions. Silylchromate-based catalysts generally produce desirable polyethylenes relative to those produced by chromium oxide-type catalysts. Silylchromate produced polyethylenes generally have a broader molecular weight distribution than those produced using chromium oxide-type catalysts. The broader molecular weight distribution leads to better processability of the resulting polyethylene. However, the productivities of silylchromate-based catalysts are typically much poorer than those realized using chromium oxide-based catalysts.
Chromium oxide-on-silica catalysts result in good production rates however certain applications require a broader molecular weight distribution.
Another disadvantage of the polymers produced with silylchromate-based catalysts is that benzene may be present in the end product which limits its application in food packaging. It is highly desirable to have a benzene free catalyst system in combination with the high productivity of the chromium oxide-on-silica catalysts and at the same time is able to produce high density polyethylene (HDPE) with broad or bimodal molecular weight distributions.